1. Field of the Invention
The present invention relates to a vehicle dynamics control system, and specifically to a system which is capable of automatically controlling or compensating for the vehicle's cornering behavior or vehicle's turning behavior or steer characteristics such as understeer or oversteer on turns by automatically properly controlling or regulating the hydraulic brake pressure applied to the individual wheel-brake cylinders.
2. Description of the Prior Art
Front-engine, front-wheel-drive (FF) vehicles, generally use a so-called diagonal split layout of brake circuits (sometimes termed "X-split layout", in which one part of the tandem master cylinder output is connected via a first brake pipeline (a first brake circuit) to front-left and rear-right wheel-brake cylinders and the other part is connected via a second brake pipeline (a second brake circuit) to front-right and rear-left wheel-brake cylinders. In vehicles with such an X-split layout of brake circuits, when the brake pedal is depressed by the driver and thus primary and secondary master-cylinder pistons are pushed, the brake-fluid pressure generated from one part of the master cylinder output and the brake-fluid pressure generated from the other part are supplied to the respective first and second brake circuits, with the result that the negative wheel torque (resulting in a braking force) is applied to the individual wheels. As is generally known, when a vehicle is rounding a curve, owing to road surface conditions (so-called low-.mu. or high-.mu. roads), changes in the vehicle velocity, throttle-on or throttle-off conditions, or the like, the vehicle may often exhibit undesired steer characteristics, namely oversteer tendencies in which the actual radius of turn is less than the intended radius of turn or understeer tendencies in which the actual radius of turn is greater than the intended radius of turn. Oversteer is generally known as an under-response to steering input as by generation of excessive slip angle on rear road wheels, whereas understeer is generally known as an over-response to steering input as by generation of excessive slip angle on front road wheels. The driver must have a great deal of skill to avoid undesired understeer or oversteer by adjusting increase or decrease in steer angle only by way of the driver's braking or steering action. For the reasons set forth above, in recent years, there have been developed and proposed various active steer-characteristics control systems in which the vehicle's cornering behavior is automatically controlled or regulated by adjusting the brake-fluid pressure applied to each individual wheel-brake cylinder by means of an electronic control unit (ECU) or an electronic control module (ECM). One such vehicle's cornering behavior controller (simply a vehicle controller) has been disclosed in Japanese Patent Provisional Publication No. 8-133039. In the vehicle controller disclosed in the Japanese Patent Provisional Publication No. 8-133039, when the vehicle experiences understeer during turns, the vehicle controller operates to build up the brake-fluid pressure in the wheel-brake cylinder of the rear road wheel rotating on the inside, thereby avoiding understeer. On the contrary, when the vehicle experiences oversteer on turns, the vehicle controller operates to build up the brake-fluid pressure in the wheel-brake cylinder of the front road wheel rotating on the outside, thereby avoiding oversteer. The Japanese Patent Provisional Publication No. 8-133039 teaches the increase in brake-fluid pressure in the front wheel-brake cylinder on the outside to avoid oversteer and the increase in brake-fluid pressure in the rear wheel-brake cylinder on the inside to avoid understeer during turns. However, on turns, the car weight is usually transferred to the outside front wheel. Particularly in front-engine, front-wheel-drive (FF) vehicles, such a tendency is remarkable since the center of gravity of the car is offset toward the front end. Therefore, the magnitude of wheel load acting on the inner rear wheel tends to become the minimum value during turns, as compared with the other road wheels. For the reasons discussed above, even if, on turns, the brake-fluid pressure in the rear wheel-brake cylinder on the inside is built up by means of the conventional vehicle controller, it may be impossible to carry out adequate braking effect, and thus it may be difficult to effectively avoid understeer tendencies owing to load transfer to the outer front wheel during turns.